1. Field of the Invention
The invention relates to soil remediation systems and, more particularly, relates to soil remediation systems having rotary drum-type treatment units and to systems for handling dust discharged from such treatment units.
2. Discussion of the Related Art
Soil remediation systems are increasingly used to treat soil contaminated with hydrocarbons or other combustible volatiles. The typical system includes at least a primary treatment unit and a baghouse. The primary treatment unit typically comprises an inclined rotary drum having a contaminated soil inlet and a remediated soil outlet at opposite ends thereof. A burner supplies heat to the outlet end of the drum.
During the remediation process, contaminated soil is fed into the drum and conveyed from the inlet end to the outlet end under rotation of the drum and heated by the burner such that the volatiles are vaporized. Hot gases produced by the remediation process entrain dust and other particulates as they flow through the drum, particularly if the gases are exhausted through the stream of incoming soil for heat exchanges purposes, and are thus heavily laden with particles when they are discharged from the exhaust outlet of the drum. A cyclone or the like is provided between the treatment unit and the baghouse to remove by gravity at least the larger dust particles entrained by the gases. The remaining particles are oxidized in an afterburner and then separated from the gases in a baghouse.
One problem encountered by the type of soil remediation system described above is that the materials removed from the gas stream in the cyclone are to a large extent still contaminated because many of the particles had been entrained by the gas stream before they were heated sufficiently to vaporize the contaminants. In fact, if the exhaust gases are drawn through the incoming materials for heat exchange purposes as discussed above, many of the particles in the gas stream will not be significantly heated before they are entrained by the gas stream and withdrawn from the drum. Heretofore, these contaminated particles were remediated through recirculation through the drum for further treatment. Such recirculation necessarily decreases the capacity of the drum and, because the particles being recirculated are very fine, a relatively high percentage of these particles are again entrained by the gases in the drum and carried out of the drum, thus further decreasing system efficiency.
Various systems have been proposed which are designed to improve the handling of dust and other particulates entrained by gas streams exhausted from remediation drums. One such system, disclosed in U.S. Pat. No. 5,193,291 to Brashears, proposes mixing the particles removed from the gas stream with the hot remediated soil discharged from the treatment drum outlet in the baghouse. Heat is transferred during this mixing from the remediated soil to the particles to vaporize the residual hydrocarbons or other volatiles, thus at least partially remediating the particles.
The system proposed by Brashears, though at least theoretically overcoming some of the disadvantages of other known remediation systems, suffers from marked drawbacks and disadvantages. Most notably, even assuming that the remediated soil discharged from the drum is still sufficiently hot when it is conveyed to the baghouse to vaporize volatiles in the particulates upon mixing, hydrocarbons and other volatiles driven from the particles escape directly to the atmosphere, thus polluting the air. Such pollution can be avoided only through costly and complicated recirculation or scrubbing procedures. In addition, the escaped hydrocarbons could condense in the baghouse and cause a fire hazard.